1. Field of the Invention
This invention relates to a method of forming an undercut microstructure, more particularly to a method involving ion implanting a substrate and etching the ion implanted substrate to form an undercut microstructure.
2. Description of the Related Art
Ridge-like microstructures formed from a substrate made from a nonlinear optical crystal, such as LiNbO3 or LiTaO3, or a semiconductor, such as Si or Ge, are widely used as optical waveguide devices of an optical integrated circuit, such as tunable microring resonators and wavelength division multiplexers (WDM) of an optical integrated circuit.
FIGS. 1 and 2 illustrate a conventional tunable microring resonator 1 formed on a nonlinear optical crystal substrate 10 of Ti:LiNbO3 (see OPTICS LETTERS, Vol. 32, No. 19 (2007) 2777 (3 pp) disclosed by the inventor of the present patent application). The conventional tunable microring resonator 1 includes a loop-shaped microring waveguide 11 protruding from the substrate 10 (see FIG. 2), and first and second couplers 12, 13 (i.e., straight waveguides) coupled respectively to two opposite sides of the microring waveguide 11. The function of the waveguide is to confine an optical field 15 of a light beam passing therethrough as much as possible. However, when the radius of the microring waveguide 11 is reduced to less than 100 μm, the geometric structure of the microring waveguide 11 causes a loss of the optical field 15 that radiates into the substrate 10. The loss of the optical field 15 limits further reduction of the radius of the microring waveguide 11 for purposes of miniaturizing the conventional tunable microring resonator 1 and achieving a highly dense optical integrated circuit.
In order to reduce the loss of the optical field 15, it has been proposed to undercut the microring waveguide 11 so as to increase a refractive index contrast at a bottom side of the microring waveguide 11 adjacent to the substrate 10, thereby increasing the optical field-confining ability of the microring waveguide 11. Conventional methods of forming an undercut under a microstructure normally involve forming a sacrificial layer between a substrate and a waveguide-forming layer with an etch rate much less than that of the sacrificial layer, followed by wet etching or reactive ion etching the sacrificial layer and the waveguide-forming layer. Since two layers of different materials having different etch rates are required, the conventional methods are complicated and time-consuming.
In addition, undercut microstructures formed on the semiconductor (Si or Ge) may also be used as parts of electronic devices, such as transistors, to achieve high frequency operation by reducing junction capacitance, or parts of electro-optic devices, such as photodiodes, to achieve improvement of carrier confinement and enhancement of quantum efficiency. U.S. Pat. No. 6,842,688 discloses a conventional method of forming a MOSFET with a T-shaped gate, which includes: forming a polysilicon layer on a gate oxide; forming an amorphous Si layer, that has an etch rate less than that of the polysilicon layer, on the polysilicon layer; forming an etched photoresist on the amorphous Si layer; and after formation of the etched photoresist, subjecting an assembly of the polysilicon layer, the amorphous Si layer, and the gate oxide to multi-step dry plasma etching so as to form the T-shaped gate on the gate oxide. The aforesaid conventional method of forming the MOSFET with the T-shaped gate is also complicated and time-consuming due to the requirement for depositing two layers of different materials having different etch rates on the substrate.